I used Parcel2go a few times, no problem and good value. Again, just a booking agent. Wheels have always ended up going through DHL, who they have recently lost a LSD that I sold, which was disappointing, but I think (hope) that is the exception. Fortunately purchaser had specifically chosen NOT to have insurance, so it was his decision, rather than me trying to save £10.
Make sure the wheels are well packaged, I sent mine in two sets of two, faces together, bubble wrap between the faces, held together with HD cable ties on the spokes, and masking tape to protect the spokes from the ties. I think it was something bizarre like £17 for four 16" wheels, without tyres.